A ”designer estrogen” pill taken to prevent brittle bones in post-menopausal women dramatically cuts their risk of breast cancer, a new study shows.

The three-year trial showed that women who took the drug, raloxifene, reduced by 76 percent their risk of developing the types of cancer associated with the hormone estrogen compared with women who took a placebo.

The drug is marketed as Evista by Eli Lilly and Co., which paid for the study. The research is published today in the Journal of the American Medical Association.

”It’s a very exciting beginning,” said Dr. Steven Cummings of the University of California at San Francisco.

”But we’ve only had 3 to 4 years of experience with this. I think women should be cautious about any medications used for prevention.”

If the results hold up over an extended period, he said, ”we’ve got an intervention that reduces the risk of breast cancer over the very long term.”

Breast cancer killed 43,500 American women last year.

Raloxifene, the first so-called designer estrogen, is part of a new generation of drugs scientists hope will mimic the good effects of estrogen – stronger bones and a lower risk of heart disease – and inhibit the harmful effects, which may include breast and uterine cancer.

But the drug isn’t risk-free. It increases the chances of serious blood clots in the lungs and legs.

Thirteen cases of breast cancer were diagnosed among the women on raloxifene; 27 cases were found among those taking the placebo.

Women taking raloxifene had a 90 percent lower risk of a type of cancer called estrogen-receptor positive breast cancer.

However, raloxifene had almost no effect on one of the difficult forms of the disease to treat – the one most commonly developed by younger women and those with a genetic predisposition.

But ”the good news is that there’s lots of options,” Cummings said. ”A year and a half ago, there was nothing we could do to prevent breast cancer. Now, we’re talking about a 50 to 75 percent, in some cases 90 percent reduction in risk.”